Add a CSS Style Guide

I started with [this blog post](https://medium.com/@fat/mediums-css-is-actually-pretty-fucking-good-b8e2a6c78b06) and revised it to more closely resemble our setup.

Peruse the current guide at [/assets/styles#readme](https://github.com/npm/newww/tree/style-guide/assets/styles#readme)
Do we want to use curly braces in our style guide examples, even though our stylesheets don't have curly braces? This is more of a nitpicky comment than anything else, but I tend to like consistency ;-)